The present invention relates to a photorefractive composite.
Photorefractive composites are potentially useful in the production of holograms for optical processing and information storage. For a review of such composites see the article by W. E. Moerner in Nature, Vol 371, pages 475 et seq (October 1994).
The photorefractive effect was first observed in inorganic materials, e.g. barium, titanate and lithium niobate. Since the demonstration of the first organic polymer based photorefractive (PR) system in 1991 [1], this class of materials has been developed to a point where they have now equalled or surpassed [2,3] many of the performance characteristics of both organic and inorganic PR crystals. Together with the low cost and versatility of organic polymer based systems this makes them highly attractive for commercial applications in optical data storage and optical data processing. Recently a PR polymer has been shown to exhibit 86% steady state diffraction efficiency [2], an outstanding increase over earlier systems, moving PR polymers further toward implementation. This composite comprises a PVK:TNF charge-transport network (which has been known for a long time and is used in photocopiers), an electro-optic polar dye (DMNPAA) and a material to lower the resulting composite viscosity at room temperature (a plasticiser) ECZ. The structures of TNF, PVK, ECZ and DMNPAA are shown below. In this prior composition, the ECZ (16% by weight) allows the re-orientation of the dye molecule at room temperature due to the lowering of the glass transition temperature (Tg), but is otherwise inert as far as the photorefractive process is concerned. This inert substance is unwanted if it is desired to achieve the highest concentration of all the active components. Several groups, however, have reported this to be a capricious and unstable system [4-6] which suffers from non-trivial sample preparation, stringent storage requirements (low humidity and dust free environment), and a risk of short device lifetimes. This system has since been reported by many groups to be extremely difficult to synthesise with good optical quality due to the crystallisation of the dye (DMNPAA) from the matrix. ##STR1##